Birds don't know about or understand glass and it kills' a lot of them. Even birds that fly away after a window collision, may later die due to the effects of a concussion.

Preventing Bird deaths from window hits is quite possible. We know what to do because 

at the Acopian Center for Ornithology at Muhlenberg College Dr. Daniel Klem and his colleagues have done the research, we just need to apply it. 

Above is a PDF from the Acopian Center detailing 

Nine Bird-window Collision Deterrent Methods 

and Relative Costs*

Which you can read and/or download if you wish.

 *Rough estimates; highly variable because of window sizes and configuration. We used a 36” high x 48” wide window for our calculations.

Note: To work well treatments must be on the outside! a Simple hawk silhouette Won't work!